Load sharing in conventional cellular spread spectrum type network systems, such as Code Division Multiple Access (CDMA), may involve load management at base stations (BTSs) of a cellular system. Load sharing may be helpful in reducing imbalances and other problems resulting from the dynamic nature of the network activity, for example, the effect of a continually changing number of communicating mobile devices, e.g., cell phones, and the transmitted power of the communicating devices. Networks that are susceptible to changing loads may suffer from a phenomenon known as “Adjacent Code Interference”, which may be handled by a power-control scheme aimed at maintaining the total power received from all mobile devices within a given cell at a generally constant level.
An additional phenomenon in the operation of a cellular spread spectrum type system, such as CDMA, is known as “adjacent cell soft-handoff”. This phenomenon relates to a procedure in which two base stations, one in the cell site where the mobile device is located and the other in the cell site to which the conversation is being transferred, are both connected to the call until a required transfer protocol is completed, at which time the call may be disassociated from the original base station. The original base station may not cut off the conversation until receiving a confirmation that the new base station at the new location has obtained control of the call.
The above described phenomena may result in a further phenomenon, known as “Cell Breathing”, which relates to the periodic expansion and contraction of cell coverage and, consequently, of the effective geographical area covered by a certain base station. Thus the cell size may continually change in response to the amount of traffic using the cell. For example, when a cell becomes extremely loaded, the cell may “shrink”, causing some of its subscriber traffic to be redirected to a neighboring cell, which may be less loaded, thereby balancing the load among cells.
Cell breathing may be encountered in 2G and 2.5G technology, and is likely to become more critical for the advanced 3G cellular systems, such as CDMA UMTS and CDMA2000, where a very high Quality of Service (QoS) to the end-user will be required to efficiently handle the types of data services introduced by such advanced systems. Also, more significant extremes between “light” and “heavy” traffic are to be expected in 3G systems, due to an inherently larger data transmission capacity.
In a multi-cell environment, the uplink capacity of a spread spectrum type network system, such as a CDMA system, may be determined by the bit energy-to-noise density ratio, Eb/No, where Eb is the energy of one bit of information and No is the total spectral noise power density, which includes both the background thermal noise and the co-channel interference caused by mobile devices in the same cell and adjacent cells.
The Eb/No system parameter typically determines the quality of the signal, e.g., a certain minimum Eb/No may be required for adequate system performance. It can be shown that as the number of mobile devices (n) increases, Eb/No, of the system decreases. Therefore, there may be a maximum number of mobile devices, n=nmax, for which Eb/No reaches its minimum value, below which value satisfactory performance of the receiver and its decoding process may not be possible.
When the number of users in the cell approaches nmax, the cell typically reaches its physical capacity limit. If this heavily loaded cell can share its load with neighboring cells by off-loading some of the users to some less heavily loaded neighboring cells, then more users may be simultaneously active within the system as a whole. Overlapping areas are important for mobile devices near the cell boundaries, where soft-handoff and counteractive fluctuations of the received signal power may be induced.
One known way to achieve load sharing is to handoff some of the users in the overlapping regions of heavily loaded cells to less loaded neighboring cells. In terms of measurable energy of radio transmission originating from the base station, the overall temporal sum of energy in a given location within the range of a particular cell increases with traffic build up. The result is that the cell “shrinking” is inversely proportional to the measured intensity.
The role of a repeater (also known as cell-extender) in a cellular network, such as CDMA, is typically to serve as an uplink/downlink signal enhancer, amplifying the received signals by a predefined gain factor. It is therefore important for proper cellular network operation that the repeater will not modify the received signals nor compress their dynamic range so as not to affect the “cell-breathing” phenomenon.
It is known that a repeater may function optimally when it is as transparent to the network as possible. A barrier to this functionality requirement may be the requirement of setting an upper limit to the downlink transmitted power. This need may be derived from environmental considerations and neighboring cell interference issues (e.g., RF coverage design) that may demand optimal power levels.
As explained above, the intensity of the RF signal at a location of a given repeater typically increases proportionally to the number of actively connected mobile devices at a given time. For this reason, a repeater that is set to amplify the uplink/downlink signals by a given gain factor, may reach or even exceed its preset power limit, occasionally reaching a range where undesired effects of nonlinear amplifiers become significant. A conventional procedure for controlling the transmitted power, known as Automatic Level Control (ALC), involves a process of automatically reducing the repeater gain when the transmit power reaches a predefined level.
Reference is now made to FIG. 1, which is a schematic graph of exemplary traffic in a conventional cellular cell as a function of the maximum permitted power output. It can be seen that the ALC process typically leads to signal compression, meaning that as the traffic in a cell drives the repeater to the plateau of the ALC, the repeater output becomes substantially constant and is no longer sensitive to changes in the repeater input power levels. If the repeater output can be prevented from reaching its non-linear saturation area, the transmit fluctuations in the repeater's input port may not be properly reflected, preventing the base station and individual mobile devices from properly operating their power control mechanisms. For example, depending on attenuation and interference, the base station may transmit control messages to the mobile devices in order to determine and set a minimum power level that meets a preset quality target. This may reduce interference with other users and may increase battery lifetime.
An additional drawback of the conventional ALC is its tendency to disturb the uplink/downlink gain balance, which is important in order for the base stations to control their associated mobile devices, e.g., to ensure that transmissions from the associated stations are received by the base stations at a similar power level, thereby preserving a generally stable Eb/No ratio for all users. Since the ALC typically operates on the downlink, the downlink/uplink gain balance; may be disrupted. This disruption may result in a reduced base-station dynamic range, reduced coverage area and over-all improper network operation due to cell breathing interruption.